I wonder what $agent is.

Otherwise,

my $mech = WWW::Mechanize->new( autocheck => 1 );
$mech->get("http://www.somesite.com/");
my %art;
for my $link ($mech->find_link( url_regex => qr/\d{6}/ )) {
    $link->url() =~ /(\d{6,})/;
    $art{$1} = 1;
}
print keys %art;
[download]

should work.

You're trying to write a C-style loop; Perl can do better.